Just FYI, since it happened to me for the first time a few weeks ago. USPS now can open Media Mail packages at the PO proper and inspect them just to make sure they are what they're supposed to be. Randomly, but it is now happening.

Since they control the opening of the package, they also control the closing, so…if it was opened when you recieve it, you know who to discuss this with, especially if it was not insured.

The USPS has always had regulations as to what may be included in various classes/categories of mail.

Sealing any piece of mail or parcel gives the USPS authorization to open and inspect.

Enclosing a note such as the "here's your books" example listed above does change the classification of the shipment.

One of my "hats" at work was shipping. There are all sorts of classifications for all sorts of products. (There is even a special one just for printed music sheets and books.) You cannot co-mingle them without causing problems for yourself.

^^That is true. Someone shipped me something in an empty alcohol (vodka, maybe) box once and, when I used to it to mail something to someone else, I was told that postal regulations prohibit mailing alcohol boxes whether they contain alcohol or not. But you guys delivered this very same box to me!